Digital amplification, by virtue of its architecture and its mode of operation, brings undeniable advantages as compared with classic analog amplification. The main advantage is its high efficiency; the heat released by a digital amplifier is minimal, enabling far more compact and hence more advantageous construction.
Another advantage of digital amplification is its appropriateness in the use of input digital signals, which naturally prevents resorting to a digital-analog conversion stage known as a “low-level” stage.
Methods and devices have therefore been proposed in the prior art, enabling this digital amplification. More specifically, two main types of digital amplifiers have been proposed, each resorting to a particular modulation method, namely respectively the types of modulation known as PWM and PDM modulation (see Table 1).
PWM-type amplifiers are the most widely used category. They generate pulses whose duration is modulated by a signal to be reproduced. FIG. 1, placed at the end of the present description, represents a simplified exemplary drawing of a standard PWM amplifier referenced 1.
The amplifier 1 comprises a modulator stage 10 proper, of the above-mentioned PWM type. This modulator 10 itself has the following elements in a cascade connection: an over-sampling circuit, typically x8, a circuit known as a pre-distortion circuit 101, a circuit called a noise shaper 102, and a PCM-PWM type conversion circuit 103. The input e1 of the PWM modulated 10 receives a PCM-encoded digital signal. These circuits are the digital part proper of the amplifier 1.
The output s10 of the modulator 10 is transmitted to a switching stage 11, forming a boundary between the digital and analog paths of the amplifier 1.
The signal, which is henceforth an analog signal, present at the output s11 of the switching stage 11 is filtered by an output filter 12 before being transmitted to use circuits (not shown) through the general output of the amplifier s1.
More specifically, the switching stage 11 may be a two-level or three-level or logic-state switching stage, this depending on the precise configuration of the modulator 10. It is generally implemented in the form of an H bridge or H semi-bridge, made as an integrated circuit or in the form of discrete components.
The output filter 12 is a low-pass filter, typically a second-order or fourth-order filter. It is designed to eliminate the switching noises from the output signal
The pre-distortion module 101 is used to counter-balance the distortion introduced by the PCM-PWM converter 103, generally known as the U.PWM type converter.
The working and characteristics, in more precise detail, of an amplifier implementing PWM technology are well known to those skilled in the art, and it is unnecessary to describe them any further.
The main disadvantage of PWM digital amplifiers is related to the production of high-frequency residues known as tonal residues which furthermore are of high amplitude. This dysfunction is due to the repetitive cycle of the PWM approach. These residues pose problems in terms of electromagnetic compatibility for certain environments in which they are used, especially for automobile-related applications.
By way of an example, FIG. 2, placed at the end of the present description, represents a frequency spectrum, ranging between 0 and 2.5 MHz, resulting from a typical implementation of the PWM amplifier of FIG. 1, when it is supplied with a high-amplitude 1 kHz sine signal. In particular, major tonal residues are seen to appear at a level close to the fundamental frequency. These residues could present problems of electromagnetic compatibility.
FIG. 3, placed at the end of the present description, is an example of a simplified drawing of a standard PDM amplifier, referenced 2.
The PDM type amplifiers have been the subject of numerous theoretical studies but there are only a few apparatuses on the market implementing this technology, all of them produced by the firm Sharp (registered mark). These amplifiers rely on the same encoding method as the SACD amplifiers (see Table 1), this method being called the DSD method (see Table 1).
The general architecture of the amplifier is similar to that of the amplifier 1 of FIG. 1: a modulator 20, receiving a PCM-format digital signal at its input e2, a switching stage 21 receiving the signal present at the output s20 of the modulator 20 and an output filter 21 receiving the signal present at the output s21 of the selector switch 21. The signal delivered at the general output s2 of the digital amplifier 2 is transmitted, as here above, to use circuits (not shown).
The basic difference between the two amplifiers of FIG. 1 and 2 lies in the nature of the modulator, namely a PDM type modulator 20 in the present case, comprising, in a cascade connection, an over-sampling circuit 200, typically x64 or x128, and a circuit known as a noise shaper circuit 201, which is typically a seventh-order circuit.
Again as here above, the switching stage 21 may be a two-level or a three-level stage, depending on the precise modulator implemented. It is generally implemented in the form of an H bridge or H semi-bridge, made as an integrated circuit or in the form of discrete components. The output filter 22 is no pass filter, typically a second-order or fourth-order filter, a low-pass filter designed to eliminate the switching noises from the output signal delivered on the general output s21.
The working and characteristics, in more precise detail, of an amplifier implementing PWM technology are well known to those skilled in the art, and it is unnecessary to describe them any further.
Although they do not have significant tonal characteristics, the high frequency residues prompted by this type of digital amplifier are very great and are endowed with high energy. This characteristic raises no problems as regards low-level conversions may but, on the contrary, represent a technological challenge for higher power values, generating an additional cost. Indeed, the high switching frequency of the output stage may be detrimental to the efficiency of the amplifier.
FIG. 4, placed at the end of the present description, represents the frequency spectrum, ranging between 0 and 2.5 MHz, resulting from a typical implementation of the PDM amplifier 2 when it is supplied with a simple, high-amplitude sine signal. It can be seen in particular that the noise level which is elevated in terms of high frequency, bears witness to a high switching frequency of the output stage.
Despite the theoretical interest represented by the use of the digital methods of the above description, it can be seen that digital amplifier implementations with digital inputs, compliant with the prior art, continue to present drawbacks which may rule out the use of the devices in certain fields of application.